Deep convective cloud (DCC) systems in the tropics produce copious precipitation and drive the global-scale circulation. Precipitation, latent heating, and cloud radiative forcing associated with DCCs are strongly modulated by cloud microphysical processes. These processes in tropical DCCs are initiated from droplet nucleation (which is determined by vapor supersaturation in updrafts and aerosol properties such as composition and size distribution. Aerosol impacts on cloud processes via this pathway are known as aerosol indirect effects, referred to as aerosol-cloud interaction in the most recent Intergovernmental Panel on Climate Change report. Aerosol impacts are a key uncertainty in understanding the current and future climate as well as extreme weather.

DCCs have complicated dynamics and microphysics; therefore, aerosol impacts on them are extremely complex and hard to disentangle. … Enhancement in DCC intensity favors enhanced storm electrification, larger precipitation rates, and taller clouds with larger anvils.

… The Observations and Modeling of the Green Ocean Amazon (GoAmazon 2014/5) experiment was carried out near the Manaus metropolis to gain a better understanding of the impacts of pollution emissions from Manaus on the hydrological cycle and climate in the tropical rainforest. … We found that the UAP_<50 introduced by the Manaus pollution plume enhanced convective intensity and precipitation rates to a degree not previously observed or simulated. The detailed simulations show that the drastic enhancement in convective intensity is primarily attributable to the enhanced condensational heating, with the latent heat released from enhanced ice-related processes at upper levels playing a secondary role. This differs from the previous “cold-cloud invigoration” concept, which does not consider aerosol impacts on condensational heating.

—Fan et al.

Effect of ultrafine aerosol particles (UAP<50) on tropical convective clouds. In clouds that lack UAP<50 (left), the clouds are highly supersaturated as a result of fast drop coalescence that forms warm rain and reduces the integrated droplet surface area available for condensation. With added UAP<50 (right, red dots), an additional number of cloud droplets are nucleated above cloud base, which lowers supersaturation drastically by enhanced condensation, releasing additional latent heat at low and middle levels, thus intensifying convection. The additional condensate adds to both the warm rain and supercooled cloud water; when freezing occurs aloft, this addition further enhances convection (i.e., a small increase in convection but enhancement of precipitation and storm electrification). Credit: AAAS, Fan et al. Click to enlarge.

The findings are based largely on unique data made possible by the GoAmazon research campaign, where scientists made ground-based and airborne measurements related to climate during 2014-2015. The campaign was run by the Atmospheric Radiation Measurement (ARM) Climate Research Facility, a DOE Office of Science user facility. Jiwen Fan of the Department of Energy’s Pacific Northwest National Laboratory (PNNL) led 21 authors from 15 institutions around the world to do the study.

We showed that the presence of these particles is one reason why some storms become so strong and produce so much rain. In a warm and humid area where atmospheric conditions are otherwise very clean, the intrusion of very small particles can make quite an impact.

—Jiwen Fan

The study capitalized on data from an area of the Amazon that is pristine except for the region around Manaus, the largest city in the Amazon, with a population of more than 2 million people. The setting gave scientists the rare opportunity to look at the impact of pollution on atmospheric processes in a largely pre-industrial environment and pinpoint the effects of the particles apart from other factors such as temperature and humidity.

In this study, scientists studied the role of ultrafine particles less than 50 nanometers wide in the development of thunderstorms. Similar but larger particles are known to play a role in feeding powerful, fast-moving updrafts of air from the land surface to the atmosphere, creating the clouds that play a central role in the formation of water droplets that fall as rain.

However, scientists had not previously observed that smaller particles below 50 nanometers, such as particles produced by vehicles and industrial processes, could do the same. The new study also revealed that these particles, the effects of which on clouds have been mostly neglected until now, can invigorate clouds in a much more powerful way than their larger counterparts.

Through detailed computer simulations, the scientists showed how the smaller particles have a powerful impact on storm clouds.

They found that when larger particles aren’t present high in a warm and humid environment, it spells opportunity for the smaller particles to act and form cloud droplets. The low concentration of large particles contributes to high levels of excessive water vapor, with relative humidity that can go well beyond 100—a key condition spurring ultrafine particles to transform into cloud droplets.

While the particles are small in size, they are large in number, and they can form many small droplets on which the excess water vapor condenses. That enhanced condensation releases more heat, and that heat makes the updrafts much more powerful: More warm air is pulled into the clouds, pulling more droplets aloft and producing more ice and snow pellets, lightning, and rain.

The result: “Invigorated convection,” as Fan says—and stronger storms.

We’ve shown that under clean and humid conditions, like those that exist over the ocean and some land in the tropics, tiny aerosols have a big impact on weather and climate and can intensify storms a great deal. More broadly, the results suggest that from pre-industrial to the present day, human activity possibly may have changed storms in these regions in powerful ways.

—Jiwen Fan

In addition to scientists from PNNL, the paper includes authors from the Hebrew University of Jerusalem, the University of Maryland, Brookhaven National Laboratory, Beijing Normal University, the Instituto Nacional de Pesquisas Espaciais in Brazil, Harvard University, the Beijing Municipal Weather Modification Office, the Universidade de São Paulo in Brazil, the Chinese Academy of Meteorological Sciences, the Federal University of Alagoas in Brazil, the Max Planck Institute for Chemistry, Johannes Gutenberg University in Germany, and Amazonas State University in Brazil.

The work was supported by the Department of Energy’s Office of Science and other organizations.

Resources

• Jiwen Fan, Daniel Rosenfeld, Yuwei Zhang, Scott E. Giangrande, Zhanqing Li, Luiz A.T. Machado, Scot T. Martin, Yan Yang, Jian Wang, Paulo Artaxo, Henrique M.J. Barbosa, Ramon C. Braga, Jennifer M. Comstock, Zhe Feng, Wenhua Gao, Helber B. Gomes, Fan Mei, Christopher Pöhlker, Mira L. Pöhlker, Ulrich Pöschl, and Rodrigo A.F. de Souza (2018) “Substantial convection and precipitation enhancements by ultrafine aerosol particles,” Science doi: 10.1126/science.aan8461